In one type of continuous wave (cw) radar ranging system, a frequency modulated signal is transmitted and directed toward a target and the signals reflected therefrom are collected and are then mixed with a portion of the modulated transmitted signal to obtain an output signal. The output signal has a frequency proportional to the distance, or range, between the transmitter and the target. As is well known in the art, the difference frequency between the frequency of the received signal and the frequency of the transmitted signal is proportional to the range of a target since the signal received back from the target is delayed in time with respect to the modulated transmitted signal being continuously produced. The amount of the delay in time is a function of the range of the target and thus the difference frequency provides a measure of target range.
As is also known in the art, microwave radar ranging systems of this type are limited in the minimum detectable target size by the size of the beam used since the signal to noise ratio (S/N) of the output signal is a function of the ratio of the cross-sectional area of the target to the cross-sectional area of the beam at the target. Thus, microwave radar systems of such type are generally not practical for obstacle avoidance applications wherein the obstacles are of relatively small size and the antenna required for detection of such small targets would be of unreasonably large size, particularly when such radar systems is used in an airborne application.
In order to detect relatively small targets, laser radar systems have been suggested because lasers may provide a sufficiently small beam having controlled divergence suitable for detecting these relatively small targets. However, in order to detect a relative small target at a reasonable range, a sufficiently high scanning rate is generally required, in order to scan or search a reasonably sized field of view. This high scanning rate requirement, however, produces at maximum range, a very short overlap time between the return signal and the instantaneous transmitted signal resulting in a relatively low signal to noise (S/N) ratio in the output signal.